1. Field of the Invention
The present invention relates to an optical device such as an optically recorded information recording/reading apparatus for reading an information signal recorded on an optical recording medium such as an optical disk.
2. Description of the Related Arts
An optically recorded information recording/reading apparatus includes an optical device which is typically referred to as an optical pickup and includes a light source such as a laser diode, so as to irradiate a recording surface of an optical recording medium with a laser beam from this light source. This optical device further captures a return beam of the laser beam reflected on the recording surface of the optical recording medium and receives the return beam on a plurality of light receiving surfaces to generate a plurality of light receiving signals. Then, these light receiving signals are combined to generate a read RF signal, a focus error signal, a tracking error signal, and the like.
In the optical system for use in the optically recorded information reading apparatus, it is known that slight reflection components remaining in respective optical parts prevent the level of the light receiving signal from reaching zero even though no recording medium is loaded and hence no return beam exists. Such remaining light is referred to as a stray light.
The existence of the stray light does not cause a grave problem in an optically recorded information reading apparatus dedicated to reproduction. However, for example, in such an optically recorded information reading apparatus that measures a pre-format characteristic of an optical disk such as a DVD-R, a DVD-RW and the like, an absolute value of a direct current level of a light receiving signal must be detected. It has been found that in such a case, the aforementioned stray light acts as a large impediment.